The invention pertains to an inductive proximity sensor with an electrical oscillatory circuit and an electrical energy source coupled thereto for compensating losses in the oscillatory circuit.
Inductive proximity sensors are generally used in automation engineering for determining operating states of automated factories, manufacturing systems (e.g., welding robots) and processing plants. Such systems use proximity switches to detect the presence or absence of electrically conductive workpieces or machine parts. Along with position determination, such sensors can also be used, for example, for rpm and speed measurements on rotating or translationally moving parts.
On the input side, inductive proximity sensors have an LC oscillatory circuit (oscillator) subject to losses, the resistance Rp of which can be influenced in a specific manner, for example by an approaching electrically conductive medium. By means of an evaluation circuit that is coupled with the oscillatory circuit, a corresponding change in the loss resistance Rp can be detected for triggering a switching signal when used as a proximity switch, for example.
An energy source coupled with the oscillatory circuit is used to compensate for the losses in order to maintain oscillations. For example, an electrical compensating current can be supplied to the oscillatory circuit. The coupled energy source acts like a negative resistance because the voltage, which drops across the oscillatory circuit, and the compensating current that is coupled back in accordance with the reference arrow conventions are oppositely directed.
With inductive proximity sensors, electromagnetic disturbances encountered in rough industrial environments can be coupled in inductively or capacitively by implementing the sensor element as an LC oscillatory circuit. Magnetic couplings effectively exert a surge in the oscillatory circuit amplitude.
In contrast, in the case of capacitive couplingsxe2x80x94depending on the phase position of the couplingxe2x80x94energy is withdrawn from or added to the oscillatory circuit, which temporarily has the effect of an unwanted change in the oscillatory circuit amplitude, because the energy that is coupled in or out capacitively can be supplied or discharged only through a loss resistance RP. The amount of energy E that is coupled in or out is also dependent on the loss resistance RP, i.e., a quality Q of the LC oscillatory circuit. The smaller the quality Q is, the better the energy can be coupled into or out of the oscillatory circuit. The time constant T for the relaxation of the disturbance that has been coupled in, i.e., the energy E that has been added, is thus dependent on the loss resistance RP of the oscillatory circuit. The higher the loss resistance, the more poorly the energy that has been coupled in is able to flow back out. In the same way, energy that is coupled out is most poorly restored when the loss resistance is low.
As a result, with known proximity sensors, coupled-in disturbances of this type can unintentionally trigger a switching signal or prevent formation of a switching signal when a proximate response element is present. Faulty switching can be successfully suppressed with the aid of presently known signal processing methods, but only with a substantial reduction of the usable signal bandwidth.
It is an object of the invention to suppress the faulty functioning of proximity sensors without reducing their usable signal bandwidth.
This object is attained for proximity sensors of the type mentioned above by coupling a filter on the input side with the oscillatory circuit and on the output side with the energy source to actively compensate for signal changes that are coupled into the oscillatory circuit. Signal changes can be caused by capacitive disturbances or by the approach of an object to be detected.
According to the invention, an additional path is provided via the filter for actively supporting the relaxation of the signal changes coupled into the oscillatory circuit. This active support of the attenuation of the signal change occurs by intermittently increasing or decreasing the amount of energy supplied by the electrical energy source to the oscillatory circuit in order to maintain the resonance oscillation as a function of the filter""s output signal.
The proximity sensor of the invention thus has the advantage that the relaxation of the signal change is substantially accelerated. As a result, the switching frequency, i.e., the frequency with which the objects can be detected, increases substantially while the relaxation time in case of disturbances is substantially shortened. Thus, the time within which an approaching object can be detected in an undisturbed manner can be lengthened accordingly.
An additional advantage provided by the invention is that the active support of the relaxation can be implemented with minimal additional complexity because the energy source that is used to support the relaxation is already present.
As has been described, it is advantageous to use the filter""s output signal in the energy source as an additional control signal for changing the energy supply to the oscillatory circuit. In other words, the relevant control input of the energy source is connected with the amplifier, which is present in any event, in such a way that the signals from the filter increase or decrease the periodic energy supply to the oscillatory circuit.
In connection with the present invention, the term xe2x80x9cfilterxe2x80x9d is used and to be understood in the broadest sense. The filter can be configured as an analog, digital electrical or electronic filter, for example, as a highpass, lowpass or bandpass filter, or as a combination of these, and it can have any desired transmission characteristics.
For the additional control of the energy supply to the oscillatory circuit described herein, the output side of the filter is directly or indirectly connected to at least one control input of the energy source. For example, the necessary signal conversion such as a voltage/current conversion for further processing of the filter""s output signal can take place between the filter output and the energy source.
On the input side, the energy source can be coupled directly or indirectly with the oscillatory circuit in order to take up the resonant frequency, and it can be connected on the output side with the oscillatory circuit in order to supply the energy maintaining the oscillations.
The filter can also be integrated into the energy source, provided the explained additional coupling path is implemented.
The filter and/or the energy source is preferably at least partially implemented as a CMOS device.